The human eye does not much sense changes in color despite variations in color temperature of illumination of a room, and this characteristic is commonly called “chromatic adaptation”. For example, moving out of a room with bluish (high in color temperature) fluorescent illumination into a room with yellowish (low in color temperature) incandescent illumination causes a white wall of the latter room to appear yellowish at first. However, passage of a short period of time causes the wall, which have continued appearing yellowish, to start appearing white.
Because of this characteristic, called “color adaptation”, of the human visual system, a change in color of illumination of a room causes an image on a television to appear different in color even when the image remains the same in color. In recent years, along with improvement in the image quality of liquid crystal televisions, there has been a growing demand for a function that enables the natural appearance of an image despite a change in color temperature of illumination of a room by changing the hue of the image according to the type of illumination of the room. The incorporation into liquid crystal televisions of a color sensor that detects the color temperature of a room is under way so that, by detecting the temperature of a room, the hue of an image can be automatically controlled to match the color adaptation of the eye. Further, in the case of a liquid crystal screen incorporated into a portable device such as a smartphone or a tablet PC (personal computer), a sensor, such as a color sensor, which automatically detects color temperature is of more importance, as the surrounding illumination changes from moment to moment depending on viewing places.
This color sensor (hereinafter referred to as “RGB sensor”) is constituted by separately sensing visible bands of R (red), G (green), and B (blue) light into which ambient light separates.
This RGB sensor uses a plurality of photoelectric conversion elements to sense ambient light, and a device that is to serve as such a photoelectric conversion element is typically constituted by a photodiode. This photodiode per se is incapable of color identification and can thus only detect the intensity of light (amount of light). Therefore, in a case of conversion of an image into an electrical signal, color identification is achieved by covering each photodiode with a color filter, detecting the amounts of R (red), G (green), and B (blue) light, i.e., three primary colors of light, with the respective photodiodes, and thereby obtaining color signals from the photodiodes.
In order to separate ambient light into three primary colors of R (red), G (green), and B (blue) light, a conventional RGB sensor typically employs a method that involves the use of a color filter that transmits or reflects particular wavelengths through the blocking of light by material absorption or the interference of light.
Meanwhile, there has been an increase in the number of two-dimensional solid-state imaging devices, such as digital still cameras and video cameras, which take images of subjects with photoelectric conversion elements composed of two-dimensional solid-state imaging elements. Moreover, color filters that transmit or reflect particular wavelengths through the blocking of light by material absorption of R (red), G (green), and B (blue) or the interference of light are mounted as on-chip filters on the pixels of a currently mainstream solid-state imaging element such as a CCD (charge-coupled device) or a CMOS (complementary metal-oxide semiconductor) imaging element, and the package is provided with an infrared cut filter, as these color filters cannot eliminate infrared light.
However, the conventional RGB sensor requires three types of photomask for the formation of a color filter that separates three primary colors of RGB light, and the requirement of these three types of photomask is a factor in increasing the time and cost of a manufacturing process.
In order to reduce the time and cost, a structure including a metal thin film subjected to nanoscale microfabrication serves as a light-wavelength-selective filter that replaces the aforementioned color filter. A light-wavelength-selective filter of this structure uses an anomalous transmission phenomenon of light by surface plasmon resonance that is excited by incident light.
Such a wavelength-selective filter using surface plasmon resonance is described in detail in PTL 1 (Japanese Unexamined Patent Application Publication No. 11-72607). There are various methods as means for causing such an anomalous transmission phenomenon, an example of which is a method of, as shown in FIG. 8, forming a filter layer 500 by forming a thin metal film approximately 50 to 200 nm thick and patterning, in the metal film 501, an array of holes 502 that are finer than a transmission wavelength. FIG. 9 shows the waveform of a spectrum that is transmitted when light is incident on the filter layer 500. Note, however, that since the surface plasmon effect is produced by the resonance between surface plasmon that is produced at the interface between a metal film and an insulator film or air and evanescent light that is produced by incident light, it is desirable, for efficient production of the surface plasmon effect, that the metal film and the insulator film be of unitary construction (material, uniformity in properties such as refractive indices, uniformity in hole pitches and shapes). For example, Au, Ag, Al, or the like is used as a metal material.
In particular, Al is often employed because it has the following advantages, for example:
(i) Al has a high plasma frequency and thus produces a resonance phenomenon even at a short wavelength.
(ii) Al is a material that is used in an ordinary semiconductor process, and requires no special apparatus or material in terms of process integration.
(iii) Al is an inexpensive material.
(iv) Al is prepared in a simple process, and enables batch formation of filters corresponding to the respective wavelengths.
Note, however, that the formation of a metal film that produces the surface plasmon effect requires microfabrication of openings at levels of 65 nm to 0.13 um under the design rule.
NPL 1 (Focus 26 <Third> Hyômen purazumon kyômei wo riyôshita karâfiruta no kaihatsu [Development of color filters using surface plasmon resonance], NIMS & Toyota Central R&D Labs., Inc.) requires a hole-to-hole pitch of approximately 200 nm for the transmission of ultraviolet light with a wavelength of 300 nm. The formation of an Al film that transmits blue light having a wavelength of approximately 400 nm requires the holes 502 to be placed at pitches of approximately 260 nm and each hole 502 to have a diameter of approximately 80 to 180 nm as shown in FIG. 10. The formation of a metal film filter that transmits light with RGB wavelengths requires the holes 502 to be placed at pitches of approximately 260 nm for blue light transmission as mentioned above.